Extracorporeal circulation device pumping system



April 5, 1955 G. V. A. MALMROS ET AL 2 Sheets-Sheet l April 5 G. v. A. MALMROS- ETAL 2,705,493

EXTRACORPOREAL CIRCULATION DEVICE PUMPING SYSTEM Filed April 9, 1952 2 Sheets-Sheet 2 E E g g O C) llVMF/VTURS GUSTAJ/MAMfil/WOS 9 00mm M7. REX & YJOH/V R mss mom ATTORNEY United States Patent EXTRACORPOREAL CIRCULATION DEVICE PUMPING SYSTEM Gustav V. A. Malmros, Binghamton, and John R. Engstrom and Donald K. Rex, Endicott, N. Y., assignors, by mesne assignments, to The Jefferson Medical College of Philadelphia, Philadelphia, Pa., a nonprofit corporation of Pennsylvania Application April 9, 1952, Serial No. 281,374 A 9 Claims. (Cl. 128-214) This invention relates generally to extracorporeal circulation devices and particularly to blood pumping systems adapted for use in devices of that nature.

Operative procedures upon the heart and its associated vessels could be better and more easily performed if the heart was temporarily relieved of its function of pumping blood. If surgical operative procedures are necessary while the heart is pumping blood, the vigorous expansions and contractions of the blood-filled pulsating member renders delicate surgical procedures extremely ditlicult, if not in many cases, a practical impossibility. Relieving the heart of its normal function of pumping blood, in whole or in part, if only for a short period of time, would of course relieve many of the attendant surgical operative difficulties. However, in order to maintain life during a temporary cessation of cardiac pumped blood flow for an appreciable time, it is necessary to assume the cardiorespiratory functions of the subject by some other means in order to maintain a life-sustaining flow of blood within the subjects circulatory system.

In general the concept of extracorporeal circulation devices is old in the art, and all devices of this type are basically aimed at temporarily assuming or assisting the functions of the heart and lungs of the subject by extracorporeal mechanical means. The mechanical means generally and broadly include a pumping system to provide a functional replacement for the heart in the pumping of both unoxygenated and oxygenated blood and a blood oxygenating system to provide a functional replacement for the lungs. In the temporary replacement of the heart and lungs and the assuming of the functions of these vital organs by a mechanical extracorporeal circulation device, it is desirable that the mechanical device approach as nearly as possible if not actually attain, an equivalent functional replacement of these vital organs. This invention is primarily directed towards a blood pumping system for extracorporeal circulation devices and a method of operating extracorporeal circulation device pumping systems, to approach an equivalent functional replacement for the heart and to permit continuous operation of blood oxygenation systems at predetermined optimum oxygenation and transfer efficiencies.

Generally the introduction of oxygen into venous blood, and the accompanying removal of carbon dioxide therefrom, in a mechanical device adapted to provide a functional replacement for the lungs, is a problem beset by many difiiculties, particularly in the case where the device is intended for use upon human beings. In order to introduce adequate and predetermined amounts of oxygen into the venous blood and to remove carbon dioxide waste products therefrom, it is necessary to provide a large surface area for oxygenating and waste product transfer purposes and to manipulate the blood flow thereon in such a manner as to form a continuous thin film of extensive surface area having sufficient turbulence of flow to provide high oxygenation and transfer efficiencies by exposing every volume of blood to an oxygen-enriched atmosphere and yet utilize only a small volume of blood at any given instant passing through the oxygenating atmosphere at relatively high flow rates without any detrimental foaming or bubbling of blood in its passage through the unit.

Many different designs of venous blood oxygenatmg devices are known to the art. One such device is illustrated and described in copending application Ser. No. 281,373, filed April 9, 1952. Most of the venous blood oxygenating units adapted for use in extracorporeal C11- Patented Apr. 5, 1955 culation devices are generally designed to operate at some optimum oxygenation and transfer efiiciency that is determined by a particular and predetermined set of operating conditions for a given unit and operate at less than the optimum efficiency under other operating conditions. The attaining of operation at optimum oxygenation efficiency is a problem that is generally dependent upon the design of the particular oxygenating unit in question, and factors such as surface area, blood film formation, blood flow characteristics, oxygen flow characteristics, damage to suspended blood cells, and removal of carbon dioxide waste material must be considered. However, the oxygenation efficiency of a given design of venous blood oxygenating unit is normally a function of, among other things, the rate of blood flow through the device. It is therefore highly desirable for maintenance of continuous operation at optimum oxygenation efliciency in any given unit to maintain a predetermined and particular rate of blood flow through the device irrespective of the rate of withdrawal of venous blood from the source thereof and the rate of return of oxygenated blood thereto, which rates should preferably also be maintained substantially equal throughout operations in order to assure maintenance of a constant volume of blood within the subject or source thereof.

In addition to the maintenance of a constant rate of blood flow through the blood oxygenating and transfer device, it is also highly desirable to maintain a constant predetermined level of oxygenated blood within the oxygenating device. The maintenance of a constant blood level within the oxygenating unit insures the maintenance of a constant volume therein and the consequent maintenance of constant volume within the source or subject and also prevents the draining of the unit with the attendant danger of drawing air bubbles into the oxygenated or arterial blood stream. To this end, it is therefore desirable to provide suitable controls to provide an automatic indication whenever the blood volume contained within the blood oxygenating device exceeds a predetermined high level and to provide a suitable warning and cessation of pumping activities when the blood contained within said unit falls below a predetermined minimum amount.

This invention may be briefly described as an improved blood pumping system for extracorporeal circulation devices which includes, in addition to venous and arterial blood pumps, an auxiliary blood pump disposed to recirculate predetermined amounts of oxygenated blood, dependent upon the amounts of venous blood being withdrawn from the source thereof, through the blood oxygenating unit and adapted to maintain a predetermined rate of blood flow therethrough independent of the rate of venous blood withdrawal and the rate of return of oxygenated blood to the subject or source thereof. The invention also includes control mechanisms responsive to the level of the oxygenated blood within the blood the design of the oxygenating unit. The maintenance of a constant rate of blood flow through a blood oxygenating unit also permits the maintenance of a constant volume of blood therein which in conventional extracorporeal circulation devices is extremely desirable because any variation of blood volume within a unit of this type must necessarily result in a detrimental variation of blood volume within the subject or source thereof.

In addition to the advantages enumerated above, an employment of the invention herein described permits a greater ease of operation of the extracorporeal circulation device by making possible the insertion of the cannulae into the subject or blood source while the vein and artery pumps are idle. This now permitted practice permits the careful priming of the blood oxygenating and transfer surface prior to actual use, the continued maintenance of a blood film on the oxygenating and transfer surfaces independent of the state of operation of the venous and oxygenated blood pumps, and in addition results in improved oxygenation and carbon dioxide transference of the blood by the continual recirculation of portions of the previously oxygenated blood.

The primary object of this invention is the provision of an improved blood pumping system for extracorporeal circulation devices.

Another object of this invention is the provision of an improved blood pumping system for extracorporeal circulation devices to permit operation of a blood oxygenating unit at constant and optimum oxygenation and carbon dioxide waste product transfer efficiencies.

Another object of this invention is the provision of an improved pumping system for extracorporeal circulation devices which permits the recirculation of portions of the oxygenated blood through the blood oxygenating device.

Another object of the invention is the provision of an improved pumping system for extracorporeal circulation devices incorporating suitable controls to maintain a constant volume of blood within a blood oxygenating unit.

Still another object of this invention is the provision of a pumping system for extracorporeal circulation devices that permits greater ease of operation of the oxygenating device and results in continual blood film maintenance and improved oxygenation and carbon dioxide transference of blood.

Other objects and advantages of this invention will be pointed out in the following disclosure and claims and illustrated in the accompanying drawings which disclose by way of example the principle of the invention and the presently preferred embodiment of the pumping system incorporating that principle.

Referring to the drawings:

Fig. 1 is a schematic plan view of the pumping system that forms the presently preferred embodiment of the invention; and

Fig. 2 is a schematic view of the pumping system illustrated in Fig. 1, together with the blood level control circuits associated therewith.

Referring to the drawings, there is provided a mounting plate to serve as support for the various components of the pumping system to be hereinafter described. Venous blood from a suitable source thereof is introduced into the device through a venous blood input tube 12. The exit end of venous blood input tube 12 is connected, at the entry side of venous blood pump 16, to at least one tube 14 passing therethrough. The venous blood pump 16 and the pumps 24, 34 and 52 which will be subsequently identified, are all modified de Bakey pumps as generally described in U. S. Patents 2,018,998 and 2,018,999. As no claim of novelty is directed towards the pumps per se, no further description thereof will be made in this application. The venous blood tube 14 is reconnected to the venous blood line 18 on the exit side of said pump 16.

To facilitate utilization of a dual source of venous blood there is also provided a second venous blood input tube 20 similarly connected to at least one tube 22cpassing through a second venous blood pump 24. The tube 22 is connected to the entry end of a common venous blood line 28 on the exit side of the pump 24 by means of a suitable connecting element 26. The connecting element 26 also connects the previously described venous blood line 18 to the entry side of the common venous blood line 28.

The exit end of the common venous blood line 28 is connected to one arm 30a of a suitable T-type connecting element 30. The venous blood contained in said common line 28 is then directed into at least one tube 32 connected to the arm 30b of said connecting element 30 and passing through the recirculating pump 34. The tube 32 is reconnected, on the exit side of the recirculating pump 34, to a common line 36 which is directly connected to the input locality of the blood oxygenating and carbon dioxide transfer unit generally designated as 40.

The blood oxygenating and carbon dioxide transfer unit 40 is schematically illustrated in the drawings and is of the general type described in detail in copending application Serial No. 281,373, filed April 9, 1952. However, it should be clearly understood that the employment of the principles of this invention is not limited to use in conjunction with the illustrated oxygenating unit 40 but finds utility in use in conjunction with any type of blood oxygenation unit.

Disposed on the underside of the blood oxygenating and carbon dioxide transfer unit 40 is an oxygenated blood output locality 42 having connected thereto a single oxygenated blood line 44 for the removal of oxygenated blood from the unit 40. The oxygenated blood line 44 is connected to one arm 46a of a suitable T-type coupling element 46. Another arm 46b of the T-type coupling 46 is connected to at least one tube 50 which passes through the oxygenated blood pump 52. The tube 50 passing through the oxygenated blood pump 52 is reconnected to the output tube 54 on the exit side of the pump 52 which tube terminates in an output coupling element 56 adapted to be connected to an exteriorly disposed tube for the return of the oxygenated blood to the source thereof. The remaining arm 460 of the T-type coupling element 46 is connected directly to the remaining amr 300 of the previously described T-type coupling 30 by a connecting tube 48 and thus provides a direct connection between the oxygenated blood tube 44 and the recirculating pump 34.

In operation of the above described pumping system, venous blood from a source or from a plurality of sources thereof, is drawn into the unit through the venous blood input tubes 12 and 20 by the action of the vein pumps 16 and 24 respectively and after passage through said pumps is directed into the common line 28 on the output side thereof. The venous blood contained within the common line 28 is directed through the T-type coupling 30 via arms 30a and 30b into the recirculating pump 34. The recirculating pump 34 pumps the venous blood via the line 36 into the input locality 38 of the blood oxygenating and carbon dioxide transfer unit 40.

The oxygenated bloodcollecting in the bottom of the oxygenator and transfer unit 40 is removed therefrom by the action of the oxygenated blood pump 52 via line 44, arms 46a and 46b and tubes 50. After passage through the oxygenated blood pump 52, the oxygenated blood is directed through line 54 in return to the source thereof.

Due to the above described inclusion of connecting tube 48 disposed betwen the arms 46c and 300 of T-type coupling elements 46 and 30 respectively, a path is provided whereby a portion of the oxygenated blood may be recirculated through the recirculating pump 34 and ahoe blood oxygenating and carbon dioxide transfer unit Each of the pumps 16, 24, 34 and 52 is driven by suitable electric motors. In operation the motor drive for the recirculating pump 34 is preset to operate said pump at a substantially constant speed that is determined by the optimum rate of blood flow through the oxygenating and transfer unit 40. Thus it is readily seen that irrespective of the rate of blood withdrawal from the subject and the rate of flow of oxygenated blood in return thereto, the maintenance of a constant operating speed for the recirculating pump 34 will result in the maintenance of a predetermined rate of blood flow through the oxygenating and transfer unit 40.

In operation of the device the speeds of the motors which drive the venous blood pumps 16 and 24 are suitably controlled so as to prevent the rate of venous blood withdrawal from the source thereof from exceeding the rate of flow through the recirculating pump 34 and oxygenating and transfer device 40, for it is obvious that if the amount pumped by the venous blood pumps 16 and 24 were permitted to exceed the amount of blood pumped by recirculating pump 34, the normal direction of blood flow in the connecting tube 48 would be reversed and as such would permit unoxygenated venous blood to bypass the oxygenating and transfer unit 40 and directly enter the oxygenated blood pump 52.

The operation of the recirculating pump 34 at a predetermined constant speed determined by the operating characteristics of the oxygenating and transfer unit 40 assures the maintenance of a constant flow rate of blood therethrough. In addition, however, in devices of this general type it is also desirable that the rate of withdrawal of venous blood from the subject be substantially balanced by the rate of return of oxygenated blood thereto in order to maintain the amount of blood within the subject as nearly as possible to a constant value. In order to assure maintenance of this equality of flow rate,

it is most convenient to maintain a substantially constant volume of oxygenated blood within the oxygenating unit 40 and to be instantly aware of any deviation in this volume from predetermined tolerances. To this end there is provided a control system that is responsive to the volume of oxygenated blood collected within the blood oxygenating and transfer unit 40.

Referring now to Fig. 2 there is provided a flat metal plate 70 embedded within the casing of the oxygenating unit 40 which is preferably constructed of plastic or other electrically non-conducting material. The fiat metal plate 70 serves as one plate of a condenser, with the oxygenated blood disposed within the unit serving as the other, and the portion of the casing of the oxygenating and transfer unit 40 disposed between said plate and the collected blood serving as a dielectric medium. The capacitance of this condenser is determined by the level of the oxygenated blood contained within the blood oxygenating and transfer unit 40 and thus provides a capacitance variation that is responsive to blood volume.

The metal plate 70 is preferably connected by a suitable lead 72 to the tank circuit of an oscillator 74 arranged so that the frequency of the output signal thereof is determined by the capacitance of the above described condenser. The oscillator and the additional circuits associated therewith are all of conventional and well-known construction and will not be described in detail. The output of the oscillator 74 is applied by a lead 76 to an amplifier 78 for amplification purposes. The output of the amplifier 78 is connected by means of a lead 80 to a tuned input circuit of a detector doubler stage 82 which rectifies the amplified alternating voltage output of the oscillator 74 and provides a direct current output voltage having an amplitude that is directly proportional to the amount of oxygenated blood disposed within the blood oxygenating and transfer unit 40. The direct current output voltage of the detector doubler circuit 82 is then applied to a stage of amplification 86 by means of a lead 84 and the amplified output thereof is applied to a conventional thyratron motor speed control circuit 90. This basic thyratron control circuit 90 is described on page 413 of volume 21 (Electronic Instruments) of the Radiation Laboratory Series, published by McGraw, Hill & Co.

The amplified direct current control voltage, the amplitude of which is proportional to the volume of blood within the oxygenating and transfer unit 40, applied to the thyratron speed control circuit 90 is thus utilized to control the speed of the oxygenated blood pump motor 96 which is preferably a direct current compound wound motor disposed within the cathode circuit of the thyratron tube within the control circuit 90.

Through the action of the above described circuit, the individual elements of which are of conventional and well known construction, the speed of the oxygenated blood pump motor 96 and therefore the speed of the oxygenated blood pump 52 is controlled directly by the level of oxygenated blood within the blood oxygenating and transfer unit 40. This speed control for the oxygenated blood pump 52 results in the maintenance of a substantially constant blood level within the oxygenating unit 40 and assures the maintenance of substantial equality between the rates of How of venous blood being withdrawn from the subject and oxygenated blood being returned thereto.

In addition to the level control circuit as described above, it is also desirable to provide an attention-arresting indication whenever the level of oxygenated blood within the oxygenating and transfer unit 40 falls below or rises above predetermined limits. To this end there is provided a flat metal plate 100 embedded in the casing of the oxygenating and transfer unit 40 and positioned so that the lower extremity thereof is disposed in line with the upper extremity of the plate 70; and a second plate 102, positioned so that the upper extremity thereof is disposed in line with the lower extremity of the plate 70. As described above in conjunction with the metal plate 70, the plates 100 and 102 form a pair of auxiliary condensers, the capacitance of which is indicative of the level of oxygenated blood whenever said level varies beyond the limits of the plate 70 and the limits of control of the level control circuit as described above.

The plates 100 and 102 are preferably connected in parallel and are connected by a suitable lead 104 to the tuning circuit of an oscillator 106 of conventional construction. The inclusion of the capacitors formed by the plates and 102 in the tuning circuit for the oscillator 106 results in an oscillator output frequency that is responsive to the blood level within the unit 40. The output frequency of the oscillator 106 is applied via lead 108 to an amplifier unit 110. The amplified output of the amplifier unit 110 is connected by lead 112 to the input circuit of a conventional detector doubler circuit 114 which rectifies the amplified alternating voltage output of the oscillator 106 and provides a direct current output developed across resistor 116 having an amplitude that is directly proportional to the level of blood within the oxygenating and transfer unit 40.

The direct current output of the detector doubler circuit 114 is applied directly via a lead 118 to a conventional stage of amplification 120 having a relay coil 122 in the plate circuit thereof. The relay coil 122 in turn has its contact points disposed within a separate circuit for a warning light 124 which will provide a vvisual indication whenever the level of blood rises above the plate 70 and within the rangs of the plate 100.

In a similar manner a portion of the direct current output of the detector doubler circuit 114 developed across resistor 116 is applied to a second conventional stage of amplification 128 by means of lead 126. Dis posed within the plate circuit of amplifier 128 is a second relay coil 130, the contact points of which are disposed in separate circuits including the oxygenated blood pump motor 96, the light 132, and a buzzer 134. Whenever the level of blood falls below the level of the control plate 70 and within the range of plate 102, the oxygenated blood pump motor 96 is stopped to prevent introduction of air into the oxygenated blood stream, and both visual and audio warning indications are provided by means of the energization of light 132 and the buzzer 134.

Thus the above described high and low level Warning control circuits provide, in addition to a cessation of oxygenated blood pumping, an attention-arresting warning to the operators of the extracorporeal circulation device who may then take suitable steps to correct the indicated departure from normal operating conditions.

In accordance with the provisions of the patent statutes, we have herein described the principle of operation of this invention, together with the elements which we now consider to constitute a workable embodiment thereof, but we desire to have it understood that the structure disclosed is only illustrative and the invention can be carried out by other means. Also, while it is designed to use the various features and elements in the combinations and relations described, some of these may be altered and modified without interfering with the more general results outlined.

Having thus described our invention, we claim:

1. An oxygenated blood recirculation system for extracorporeal circulation devices having a venous blood oxygenating unit with a venous blood input locality and oxygenated blood output locality, means for pumping venous blood from a source thereof into said unit and means for pumping oxygenated blood from said unit in return to the source thereof comprising recirculation pumping means disposed between said venous blood pumping means and said oxygenating unit for pumping the venous blood output of said venous blood pumping means into said oxygenating unit and blood transmitting means connecting the oxygenated blood output locality of said oxygenating unit with said recirculation pumping means for permitting recirculation of portions of the oxygenated blood through said oxygenating unit.

2. An oxygenated blood recirculation system for extracorporeal circulation devices having a venous blood oxygenating unit with an oxygenated blood output locality, means for pumping venous blood from a source thereof into said unit, and means for pumping oxygenated blood from said unit in return to the source thereof com.- prising a recirculating pump disposed between said venous blood pumping means and said oxygenating unit for pumping the venous blood output of said venous blood pumping means into said oxygenating unit and means connecting the oxygenated blood output locality of said oxygenating unit with said recirculation pump for recirculating a portion of the oxygenated blood through said oxygenating unit.

3. An oxygenated blood recirculation system for extra corporeal circulation devices having a venous blood oxygenating unit, means for pumping venous blood from a source thereof into said unit and means for pumping oxygenated blood from said unit in return to the source thereof comprising an auxiliary pump disposed between said venous blood pumping means and said oxygenating unit for pumping the venous blood output of said venous blood pumping means into said oxygenating unit and blood transmitting means connecting said oxygenating unit with said auxiliary pump for permitting recirculation of portions of oxygenated blood through said oxygenating unit.

4. In an extracorporeal circulation device having a venous blood oxygenating unit, means for pumping venous blood from a source thereof into said unit and means for pumping oxygenated blood from said unit in return to the source thereof, an oxygenated blood recirculation system comprising an auxiliary pump disposed between said venous and oxygenated blood pumping means and said oxygenating unit for pumping the venous blood output of said venous blood pumping means into said oxygenating unit and recirculating a portion of the oxygenated blood through said oxygenating unit.

5. In an extracorporeal circulation device, having a venous blood oxygenating unit, means for pumping venous blood from a source thereof into said unit having an input side and an output side and means for pumping oxygenated blood from said unit in return to the source thereof, said means having an input side and an output side, an oxygenated blood recirculation system comprising an oxygenated blood recirculating pump disposed between the output side of said venous blood pumping means and said oxygenating unit and connected intermediate said oxygenating unit and the input side of said oxygenated blood pumping means for pumping the venous blood output of said venous blood pumping means and a portion of the oxygenated blood output of said oxygenating unit through said oxygenating unit.

6. A blood pumping system for extracorporeal circulation devices having a venous blood oxygenating unit comprising at least one venous blood pump for withdrawing venous blood from a source thereof and introducing it into said oxygenating unit, at least one oxygenated blood pump for withdrawing oxygenated blood from said oxygenating unit in return to the source thereof, and an auxiliary pump disposed between said venous and oxygenated blood pumps and said oxygenating unit for pumping the venous blood output of said venous blood pump and portions of said oxygenated blood into said oxygenating unit.

7. A blood pumping system for extracorporeal circulation devices having a venous blood oxygenating unit comprising at least one venous blood pump for withdrawing venous blood from a source thereof and introducing said blood into said oxygenating unit, at least one oxygenated blood pump for withdrawing oxygenated blood from said oxygenating unit in return to the source thereof, and an auxiliary pump disposed between said venous and oxygenated blood pumps and said oxygenating unit arranged to pump the venous blood output of said venous blood pump and portions of said oxygenated blood into said oxygenating unit for maintaining a substantially constant flow rate therethrough.

8. A blood pumping system for extracorporeal circulation devices having a venous blood oxygenating unit, at least one venous blood pump for withdrawing venous blood from a source thereof and introducing said venous blood into said oxygenating unit, and at least one oxygenated blood pump for pumping oxygenated blood from said oxygenating unit in return to the source thereof, comprising an auxiliary pump, disposed between said venous and oxygenated blood pumps and said oxygenating unit arranged to pump the venous blood output of said venous blood pump and portions of said oxygenated blood into said oxygenating unit for maintaining a substantially constant flow rate therethrough.

9. A blood pumping system for extracorporeal circulation devices having a venous blood oxygenating unit with a predetermined optimum flow rate characteristic, at least one venous blood pump for withdrawing venous blood from a source thereof and introducing said blood into said oxygenating unit, at least one oxygenated blood pump for withdrawing oxygenated blood from said oxygenating unit in return to the source thereof comprising, an auxiliary pump disposed between said venous and oxygenated blood pumps and said oxygenating unit arranged to pump the venous blood output of said venous blood pump and portions of said oxygenated blood into said oxygenatingunit for maintaining said optimum rate of flow therethrough.

References Cited in the file of this patent UNITED STATES PATENTS 2,607,828 Razek Aug. 19, 1952 FOREIGN PATENTS 546,947 France Sept. 6, 1922 OTHER REFERENCES Gibbon, The maintenance of Life During Experimental Occlusion of the Pulmonary Artery Followed by Survival, Surgery, Gynecology and Obstetrics, vol. 69, November 1939, pages 602-614. (Copy in Division 55.) 

1. AN OXYGENATED BLOOD RECIRCULATION SYSTEM FOR EXTRACORPOREAL CIRCULATION DEVICES HAVING A VENOUS BLOOD OXYGENATING UNIT WITH A VENOUS BLOOD INPUT LOCALITY AND OXYGENATED BLOOD OUTPUT LOCALITY, MEANS FOR PUMPING VENOUS BLOOD FROM A SOURCE THEREOF INTO SAID UNIT AND MEANS FOR PUMPING OXYGENATED BLOOD FROM SAID UNIT IN RETURN TO THE SOURCE THEREOF COMPRISING RECIRCULATION PUMPING MEANS DISPOSED BETWEEN SAID VENOUS BLOOD PUMPING MEANS AND SAID OXYGENATING UNIT FOR PUMPING THE VENOUS BLOOD OUTPUT OF SAID VENOUS BLOOD PUMPING MEANS INTO SAID OXYGENATING UNIT AND BLOOD TRANSMITTING MEANS CONNECTING THE OXYGENATED BLOOD OUTPUT LOCALITY OF SAID OXYGENATING UNIT WITH SAID RECIRCULATION PUMPING MEANS FOR PERMITTING RECIRCULATION OF PORTIONS OF THE OXYGENATED BLOOD THROUGH SAID OXYGENATING UNIT. 